How can I create reasonably challenging encounters for large groups?

Question

I'm running a 5e campaign that has become quite successful to the point that everyone wants in and I have 6-8 players at my table every other week.

It's awesome, but I'm having troubles with creating challenging encounters. Two of the players are very heavy crowd-controllers (a Lore Bard and a Divination Wizard). The divination wizard also uses his portent to force fails in important saving throws for abilities like Suggestion or Hold Person on the strongest enemies, effectively dismantling my encounters.

That wouldn't be that big a problem if there weren't 4-6 more players backing them up with heavy damage to deal with the lower-challenge enemies.

They aren't even overly geared. Some of them have only 1 uncommon item (+1 weapon), a couple of them have a couple extra uncommon items that do not affect combat (eyes of minute seeing, goggles of night, etc)

So my question is: How can I keep my encounters challenging without adding a million enemies? I've noticed that if my encounter features 8+ enemies, each round becomes very slow and tedious (16+ turns per round).

As an example, the last fight of my last game was against 4 enemies with CR 6 (this adds up to 9200 exp, which is between the Hard and the Deadly difficulty thresholds for this party). The entire group was paralyzed by Hold Person pretty much the entire fight, and I was at a loss as to how to handle this situation. Perhaps it was OK that this happened?

Edit: Thanks for the replies! The problem seems to be having a low amount of encounters. On that in-game day the party only had 2 hard encounters. I asked some of the players and they were all pretty much out of spell slots by the end of that fight, which means that anything else I threw at them would cost them dearly.

Answer 1

It sounds like you have two problems: (1) your game with 6-8 players has slow combats unless you use a small number of monsters, and (2) if you use a small number of monsters, your crowd-controllers can shut the encounter down.

The first problem is easier to fix:

• Use a larger number of monsters, but make most of them them identical so that you can run turns for them quickly. Move them all on the same initiative. "Okay, these three orcs are attacking you. Each of them makes one attack, with an attack bonus of +4... (rolls) they hit AC 11, 15, and 17. How many hits? Okay, take... (rolls) 12 damage." Optionally include one or two bigger monsters so that your crowd-controllers have something awesome to do.
• Consider splitting your game into two smaller games. You can start slowly -- have occasional "side missions" which don't involve the whole group. These will be easier to schedule: choose the time and place that work best for you, and see if around half of your players are able to be there. If that seems to work, you can have more and more side missions, and save the "full group" scenarios for boss fights and special occasions. In the extreme case, this turns into a West Marches scenario.

For the second problem: remember that D&D 5e is balanced around having a lot of encounters per day. Are you doing that? If you're only giving them one big fight in between rests, your characters with daily powers will be much more powerful than the designers intended, because they can use all their dailies in that encounter. Those crowd-control abilities: are they daily powers?

If you suddenly change your game from one fight per day to three or four, make sure to telegraph that in advance so your casters don't waste all their spells on the first fight.

One final note: it can be dangerous to have battles with a small number of active monsters. What tends to happen is each monster stands in one place and focuses all its damage on one player character. This is really bad for that player character, and it's sort of boring for all the other characters who never get attacked. One solution I've used for this problem is, when I have an encounter with just one monster, I make sure that all its attacks are area-effect attacks, so that it spreads out the damage more evenly. This involves a lot of inventing homebrew monsters, though, so it may not be best for every group.

Answer 2

The problems you're discussing are very common across RPG groups. We can break the issues down between 'combat' (hold person) and 'non-combat' (suggestion) encounters.

How Do I Make Combat Encounters More Challenging (without increasing the number of monsters)

Use more powerful monsters

This one should be relatively obvious, as the higher the CR of the monster, the more difficult (in theory) it is to defeat. If you find that your party is steamrolling your 4 CR6 monsters, start looking at having fewer, higher-level monsters. As the CR increases, the lethality and number of abilities the monsters have quickly increase. A CR8 monster can do things that a CR6 can only dream of, and that can give your party a lot to mull over when they're fighting one.

Adapt the monsters to your party's power level

Your Divination Wizard is using hold person every chance they get? Well, the BigBad realized this and sent his non-humanoid lieutenants after the party (hold person only works on humanoid creatures). Your Sorcerer is dropping fireballs like its their job? The now-terrified king of the region sent some fire-elementals to ruin their plans. Your Barbarian is crushing everything within its reach with a hammer? A few flameskulls should give them something to think about.

Customize the monsters you were going to use anyways

The evil wizard invented a tattoo that, when applied to humanoid characters in the proper fashion, gives them advantage on Wisdom saving throws (and makes it much more difficult to Hold Person or Suggest them). The Orc warchief becomes enraged when burned. The ultra-strong giant that defends the entrance to the cave has been imbued with a special power that gives him legendary resistance.

More encounters per day

This is probably the biggest one. The DMG will recommend you put your party through 8 encounters per long rest. However, most normal people do 2-3 encounters per day, which makes spellcasters extremely powerful. Your Divination Wizard gets 2 portent dice a day. If he has to balance that between 8 combat encounters and 6 social encounters, then there will be many times when there just aren't any portent dice left to ruin your encounter. However, if there are only a few encounters per day, then your Wizard will likely be able to use portent whenever they need to, which is going to derail your plans.

How Do I Make Non-Combat Encounters More Challenging

Have your NPCs start adapting to the players

By the second or third time the BigBad's plans are derailed because of a well-placed Suggestion spell, they should start putting contingency plans into action. Sure, they may not be able to work out that it was specifically Suggestion that was being cast, but eventually they're going to realize that someone is able to force previously-loyal minions to perform actions. A smart enemy would start sowing misinformation, creating tests (tell each of your top 10 minions that the Sword of PCDesire is hidden in a different location. Whichever location the PCs raid corresponds to the minion they were able to charm), and generally making Suggestion less powerful.

Fudge it

This is a divisive issue, but when low-level PCs get access to spells as powerful as Suggestion, sometimes you just have to fudge it. The king of the land rolled a nat 1 on their save? Maybe it was actually a 15, so they passed. The group manages to cast Suggestion on the one NPC that knows the location of the BigBad's lair? Well, maybe he only thought he knew. Maybe he didn't actually know at all, it was the guy standing next to him who knew, and you just mixed up your notes. Maybe the BigBad had been planning to move for months, and just never got around to telling this guy about the changes.

Final Thoughts:

I want you to ask yourself two questions before you start doing any of this.

1. Am I prepared for this to get worse as time goes on?

High-level PCs are a force to be reckoned with. 8 high-level PCs are going to cause you some major pain. There is always going to be something that they uncannily excel at, and short of re-writing reality you're probably going to have to deal with it. To make matters worse, they're naturally going to gravitate to the things they do well and shy away from the things they do poorly. You haven't stated their min-max attitude, but if they metagame their characters at level-up time and before/after combat, it'll be near impossible to devise consistently challenging encounters that don't simultaneously run the risk of one-shotting someone, or having an unlucky roll turn into a TPK. In that case, you'll likely be forced to simply increase the number of encounters per day, and then taper the encounters off once the party starts getting worn out.

2. Do my players see a problem with the current setup?

I played with a group that absolutely loved to crush combat encounters. The more challenging the encounter, the less fun they had. They didn't care how easy it was, or how short rounds were. They just wanted to win, and win hard. That's not to say that your group is thinking that way, but are you sure that the players want it to be more challenging? If they do then go right ahead, but before you go through all this work, make sure the players will actually appreciate it.

Answer 3

Some tactics I've used sometimes when one or two player characters develop 'overpowered' strategies:

Throw it back at them

So your crowd controllers are essentially locking down encounters? Let them have a couple of encounters facing cult members with access to some of the same spells they're using. When the barbarian suddenly decides to go take a dip in a nearby acid pit (suggestion is a pain sometimes) and all of his saving rolls are failing, your PC's will find that they have to improvise in awesome ways in order to avoid the issue. You can then steal their tactics for the next time they try using that strategy, forcing them to invent new plans.

If you're having issues with 8 players all focusing on one enemy, throw it back at them! Use swarm creatures and have them focus on the cleric, or treat several creatures as 'one' (with one initiative as suggested in Dan B's answer) and start hammering away at their barbarian. Again you can use their tactics and methods for dealing with it as a cue for further problem avoidance.

Invent something that's unexpected in the environment

Suggestion becomes useless in burning buildings, for example, if everyone is taking 1 damage per turn from the heat (the 'snap out of it' effect).

Similarly: it doesn't matter how big the group is (and in fact it can become a problem for the players to overcome) if the combat zone is slowly filling up with water, or being overgrown by magical vines, or is actually set on a series of minecarts the players have to co-ordinate in order to get within combat range. There are any number of ways to divide and conquer (well, not conquer, but you get what I mean) using funky environmental effects.

Invent something unexpected for the monsters to do

This one I tend to be careful with, and I'll playtest myself before throwing it at the group, but sometimes adding a single unexpected ability can completely change the way a combat situation goes. For example:

Four skeletons attack the group. Three are instantly dismembered, one is held in place using hold person and then dismembered the next turn. The players start to walk over the skeletons when dun dun dun sets of the shattered bones recombine to form two slightly larger, more buff skeletons. Again, one gets held down and crushed, and the other does damage to the barbarian and cleric before it goes down. Then dun dun duuuuuun!! their broken corpses recombine and then burst into flame! There is no more 'hold person'. The barbarian (who went all out to start with) is almost out of juice, the cleric has used their daily power. What started out looking like an easy kerbstomp has turned into a challenging encounter.

The above requires a lot of playtesting to get right, but essentially combines 3 combat encounters (4x broken skeleton, 2x skeleton, flaming skeleton) to keep the turn rate up but provide a challenge. Other examples include but are not limited to:

Enemies that aren't on the battlefield all the time (works well with 7 or 8 ghosts phasing in in pairs).

One monolithic enemy that keeps growing and gains more attacks with size (slimes and cubes are good for this).

Challenges that take place inside combat encounters (shut down the golem factory as it keeps spitting out Warforged Bards)

The grind of these encounters will quickly remove your player's OP abilities, while also allowing you to avoid TPK by keeping the actual challenge level fairly balanced.

Answer 4

There are several possible solutions to this issue, and I recommend a mix of all of them depending on the specific encounter at hand. In no specific order:

Make Enemies Larger

This is so standard a GM technique that it doesn't have to be discussed. Bigger enemies are generally more dangerous than smaller enemies, though some (such as lichs) fit the spirit of the trope rather than the literal meaning.

Make More Enemies

Yes, this can pose problems with pacing, but take a cue from Fantasy Flight's Star Wars games with the minions mechanic. A group of 3-5 enemies might work together as a unit, flanking characters and isolating them while using the same initiative slot. The way I would treat a situation like this is using a single attack roll, but giving bonuses pertinent to surrounding PCs if applicable. You might even give other bonuses, but the biggest use for this is adding survivability to the mobs without adding more rolls. This also combos well with:

Make Enemies Smarter

Remember that players defeat enemies far more powerful than themselves not by fighting harder, but by fighting smarter. Enemies with any semblance of intelligence might also be able to avail themselves of this stratagem, and it can manifest in various ways with the previous two techniques. Superior numbers acting in tightly-knit groups can more easily isolate powerful, but squishy casters from their sturdier allies, or focus fire at range to accomplish the same effect. Larger enemies may be most dangerous when they act as commander, organizing the mobs to crush the adventurers, though this practice can prompt prepared parties to simply silence the commander. Which can lead to the next option:

Make Enemies Trickier

Rather than just using tactics, let the enemies specifically exploit the natural tendencies of the party. Perhaps the party sees a goblin clan apparently led by a towering orc bellowing orders; they party might attempt to incapacitate the larger enemy, playing right into the true leaders' hands. In reality, the two chieftain assassins are preparing to backstab the two most dangerous members of the party, the orc being merely a distraction to buy time for the true leaders.

Make Use of Environment

Traps, hidden ambush sites, escape hatches, all allow for otherwise weaker enemies to stand up to high-level parties. In the thick of battle, what are the chances that every member of the party will manage to avoid a tripwire?

Summary

The combinations of those five tactics will greatly improve the variety of combat. Using Hold Person on the most dangerous-looking foe will be less useful when the real threat is hidden. Even without trickery like that, the minions rule I suggested means that Hold Person becomes much less useful when two more of the enemy are still able to function, and the weaker groups are able to hold out longer against the 4-6 other players.

Answer 5

One alternative increasing the number of creatures is to find equivalent or like monsters with a higher Challenge Rating.

Ex. So instead of goblins, try hobgoblins.

Another alternative is to simply give creatures more hit dice, which in turns means more hit points, this however descales the expectations of players and characters who have experienced these creatures already. You'll have to explain why they are tougher. I prefer to find creatures with a greater challenge rating to make the encounter tougher without adding the number of turns per round of combat.

Answer 6

Get Help:

For really big encounters, with lots of players facing powerful opponents, I would have another person help run the bad guys. This "assistant GM" had the freedom to think up clever ways to inconvenience the players, freedom I didn't have because I was focussing on all of the other game details.